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differ in the area under scrutiny. Documentation of this information should be made 
available for review as part of the 303(d) listing cycle for which a new subdivided 
segment is initially assessed. Jurisdictions need to ensure that any sub-segmentation 
is fully consistent with their state’s water quality standard regulations. 
The EPA discourages states from subdividing segments simply to remove smaller 
areas from an impaired waters list. Given the tidal exchange that occurs among all 
segments, conditions in one segment can potentially affect adjacent segments. A sub- 
segment that is prematurely removed from the impaired waters list might require 
placement back on the impaired waters list in the next listing cycle due to adverse 
conditions in the original segment. 
Maryland and Virginia have already adopted specific sub-segments into their state’s 
water quality standards regulations in several tidal tributaries and embayments. The 
2004 addendum to the 2003 Chesapeake Bay use attainability and designated-use 
document contains detailed documentation supporting these state-defined, adopted 
sub-segmentations (U.S. EPA 2004c). 
DATA FOR USE IN CHESAPEAKE BAY 
CRITERIA ASSESSMENTS 
To assess Chesapeake Bay water quality criteria attainment, the data used must prove 
adequate. Consistent with the 2003 EPA Chesapeake Bay criteria assessment guid¬ 
ance, the data should be of known quality and adequate quantity, as well as 
representative of the tidal water designated use habitat under assessment (U.S. EPA 
2003a). Documented QA/QC programs should ensure data quality; such documen¬ 
tation should be publicly available for evaluation. A sufficient amount of data should 
exist to provide a defensible degree of accuracy and precision given the expected 
level of variability in the assessed tidal water body. The data should also be repre¬ 
sentative of the spatial assessment unit as a whole so the resulting assessment is not 
biased toward any one portion. While the EPA provides no minimum requirements 
for each of these data characteristics, they should be maximized to the extent 
possible to ensure that criteria assessments are scientifically defensible. 
Opinions range broadly on the quantity of data required for criteria assessment. On 
one extreme, some believe that sufficient data should be collected to capture all the 
temporal and spatial variability to ensure that the criteria and designated uses are 
attained in space and time. On the other extreme, some suggest that the state agency 
manager should determine if a designated use is being attained based on available 
information—even if it is anecdotal. 
For the Chesapeake Bay and its tidal tributaries, the EPA recommends basing all 
water quality criteria assessments on monitoring data. These data should be collected 
over a three-year period immediately prior to the year of the listing cycle, unless 
non-attainment is definitively established in less time (as described in Chapter 7). 
Furthermore, the monitoring program for data collection should optimize quality, 
quantity, and representativeness as described above. 
I 
chapter iii • Application of Chesapeake Bay Water Quality Criteria Assessment Procedures 
